underestimate the real value or ability of; "Don't sell your students short--they are just shy and don't show off their knowledge"
sell securities or commodities or foreign currency that is not actually owned by the seller, who hopes to cover (buy back) the sold items at a lower price and thus to earn a profit
to believe especially on uncertain or tentative grounds; "Scientists supposed that large dinosaurs lived in swamps"
talk over conjecturally, or review in an idle or casual way and with an element of doubt or without sufficient reason to reach a conclusion; "We were speculating whether the President had to resign after the scandal"
invest at a risk; "I bought this house not because I want to live in it but to sell it later at a good price, so I am speculating"